You can use the indent-rigidly
command to do this. I would however not recommend disabling markdown-mode's standard TAB binding since it does much more than indenting (for exemple it cycles visibility when point is on a header line)
For example, put this in your initialization file to bind C-TAB to rigid indentation:
(eval-after-load "markdown-mode" '(define-key markdown-mode-map (kbd "C-<tab>") (lambda () (interactive) (indent-rigidly (region-beginning) (region-end) 4))))